Embodiments of the present invention relate to a method for transmitting a signal. Further embodiments relate to a method for receiving a signal. Further embodiments of the present invention relate to an apparatus for transmitting a signal. Further embodiments of the present invention relate to an apparatus for receiving a signal.
In the field of multiple access algorithms via so-called shared-media channels the so-called Single-Carrier Frequency-Division Multiple Access (SC-FDMA) has gained acceptance in mobile communications in the Long Term Evolution (LTE) for the uplink from the terminals to the base station. This method provides a number of advantages, including: a high spectral efficiency, a highly flexible assigning of resources according to a channel quality and network utilization, simple means for equalization in the frequency domain and low power fluctuations compared to Orthogonal Frequency-Division Multiple Access (OFDMA), so that the single-carrier frequency-division multiple access (SC-FDMA) scheme is already used in the Long Term Evolution (LTE) mobile radio standard [4].
The last point, the low power fluctuation, was not identified as a core problem in LTE Release 8-10. However, meanwhile this topic is focused on increasingly, because a strong increase of the number of users of so-called Machine-To-Machine Communication (M2M) is expected. This is taken to mean a connection of innumerable distributed sensors via mobile communication to the internet. This raises the question, how to integrate mostly low level services for sensor nodes as efficiently as possible into mobile communications with the lowest energy or power consumption possible.
Presently, the relatively high overhead of the protocol is initially focused, which is necessitated for a random access such as a dial-up of a sensor node to the mobile network. However, a majority of the sensors will be immobile. For regular operations, fixed, i.e., constant or time invariant, channel resources may be assigned and primarily the transmit power has to be utilized efficiently to achieve long battery operation times.
For quite some time an approach is examined, to extend the SC-FDMA method by an additional root-raised-cosine filtering, whereby a higher bandwidth is used for transmitting a signal. The filtering leads to a smoothing of the envelope as it is described by a method 100 in FIG. 1. The output signal of the Discrete Fourier Transformation (DFT) is repeated several times in the frequency domain to emulate an oversampling in the time domain. Then, the signal is going to be filtered in the frequency domain, the filter having a root-raised-cosine profile. The equalization that can be achieved depends on the so-called α-factor of a filter profile. The larger it is, the more bandwidth is necessitated and the smoother or plainer is the envelope of the waveform. A smooth or plain envelope means low fluctuations of an amplitude of the envelope of a waveform, as it will be described in FIGS. 5a-5d. While in [7, 16, 17] α-values below 0.25 were examined, values of about α=0.7 effectuate a significantly smoother envelope, as it is described in [11, 19]. To achieve this, also a higher bandwidth is necessitated.
A patent [9] as well as several session publications, which are summarized in a journal article [10] cover the CPM-IFDMA proposal. These publications cover combinations of the CPM with the distributed form of SC-FDMA.
Continuative problem solving approaches are based on the so-called continuous phase modulation (CPM) as it is described exemplarily in [9, 10]. The solution proposed up to now is based on a second, distributed variant of SC-FDMA, which is also denoted as Interleaved Frequency Division Multiple Access (IFDMA). During this transmission, a comb is formed in the frequency domain out of fractions of the altogether available OFDM-subcarriers with a constant gap between the used subcarriers. IFDMA has not gained acceptance in LTE for several reasons, in particular the high pilot overhead necessitated for channel estimation. In addition, the CPM-IFDMA approach seems to be not completely optimized with respect to the remaining power fluctuations. The remaining fluctuations, described in [10] are supposedly to be traced back to the generation of the CPM-sequence before performing the N-DFT. In [9, 10], the MSK modulation is generated in the time domain, subsampled and afterwards routed in the N-DFT.
Hence, there is a need for an optimization of transmit power efficiency, which allows for an implementation to or an integration into the actually used localized SC-FDMA transmission mode in LTE. An increased transmit power efficiency may extend battery lifetime of a sensor unit communicating via a wireless or wired network.